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air turbulent expanding premixed flame
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Experimental investigation on the self-acceleration of 10%H2/90%CO/air turbulent expanding premixed flame Guo-Peng Zhang a,b, Guo-Xiu Li a,b,*, Hong-Meng Li a,b, Yan-Huan Jiang a,b, Jia-Cheng Lv a,b a
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing, 100044, China Key Laboratory of Vehicle Advanced Manufacturing, Measuring and Control Technology (Beijing Jiaotong University), Ministry of Education, Beijing, 100044, China
b
highlights Effect of turbulence intensity on turbulent burning velocity was studied. Nonlinear propagation characteristics of turbulent premixed flame were evaluated. Fractal and acceleration characteristics in the transition stage were studied.
article info
abstract
Article history:
The self-acceleration characteristics of a syngas/air mixture turbulent premixed flame
Received 21 May 2019
were experimentally evaluated using a 10% H2/90% CO/air mixture turbulent premixed
Received in revised form
flame by varying the turbulence intensity and equivalence ratio at atmospheric pressure
19 July 2019
and temperature. The propagation characteristics of the turbulent premixed flame
Accepted 22 July 2019
including the variation in the flame propagation speed and turbulent burning velocity of
Available online 10 August 2019
the syngas/air mixture turbulent premixed flame were evaluated. In addition, the effect of the self-acceleration characteristics of the turbulent premixed flame was also evaluated.
Keywords:
The results show that turbulence gradually changes the radius of the premixed flame from
Syngas
linear growth to nonlinear growth. With the increase of turbulence intensity, the formation
Turbulent premixed flame
of a cellular structure of the flame front accelerated, increasing the flame propagation
Flame propagation speed
speed and burning speed. In the transition stage, the acceleration exponent and fractal
Turbulent burning velocity
excess of the turbulent premixed flame decreased with increasing equivalence ratio and
Self-acceleration characteristic
increased with increasing turbulence intensity at an equivalence ratio of 0.6. The acceleration exponent was always greater than 1.5. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China. E-mail address: [email protected] (G.-X. Li). https://doi.org/10.1016/j.ijhydene.2019.07.154 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction In view of increasingly severe environmental problems and energy shortages, two important solutions are the exploration of clean and reliable alternative energy sources and the elucidation of combustion mechanisms to achieve efficient fuel combustion [1e6]. Syngas, a mixture of CO and H2 as the main components, has many sources and can be produced by a variety of methods. It can be prepared by the gasification of a solid fuel such as coal or coke [7], or light hydrocarbons such as methane and propane [8]. It can also be partially oxidized with macromolecular hydrocarbon fuels such as diesel and kerosene [9]. As an important component of syngas, H2 has the advantages of fast flame propagation, low ignition energy, and wide combustion limit [10]. Therefore, syngas has been widely investigated as an alternative fuel for internal combustion engines [11e13]. The turbulent expanding premixed flame is a major form of combustion and a typical form of flame in the spark-ignited internal combustion engines [14]. The turbulent expanding premixed flame has been investigated in detail [15,16]. Different turbulent expanding premixed flame combustion characteristics have been proposed, because of the difference in fuel [17], turbulence intensity [18], equivalence ratio [19], and the range of observation radius [20]. Gas-fueled engine suffers from combustion cycle variation. Studies have shown that the combustion cycle variation of an engine is mainly caused by a turbulent flow acting on flame propagation [21]. Therefore, it is important to elucidate the mechanism of the propagation characteristics of the turbulent premixed flame of syngas for optimizing the combustion process of power machinery and design of combustion system. Pierre et al. [22] evaluated the effect of turbulence intensity and pressure on the flame propagation speed in a spherical turbulent premixed flame and used them as a radius or stretch function to better understand the dynamic characteristics of flame propagation. Li et al. [23] evaluated the distribution of the local radius of flame and the local propagation speed of flame at different turbulence intensities of an equimolar mixture of H2 and CO and elucidated their relationship. Turbulence is known to increase the disturbance of the flame front and enhance the fluctuation of local radius of flame, thus increasing the fluctuation in the local propagation speed of the flame. Goulier et al. [24] reported the effect of the turbulent flame speed correlation on the turbulent combustion characteristics of a lean to stoichiometric H2/air mixture. In addition, Wang et al. [25] evaluated the correlation of turbulent burning velocity under a high pressure of syngas premixed flame. Chiu et al. [26] studied the turbulent burning velocity of a highpressure syngas under constant Reynolds number. A turbulent premixed flame undergoes an accelerated propagation process. On one hand, the flame inherent instabilities of a flame makes the flame front produce a cellular structure and accelerate flame propagation. On the other hand, the scale of turbulent flow effect on flame becomes larger, turbulent disturbance is enhanced, and flame propagation is accelerated. At present, many studies do not consider turbulent flow, but only focus on the effect of flame inherent instability on accelerated flame propagation [27e31]. Yang et al. [32] evaluated the effect of flame inherent instability on the flame
propagation and self-acceleration in a laminar flow environment. The formation of a cellular structure on the flame front is dominated by thermal diffusive instability, and the flame front develops to saturation region by hydrodynamic instability. Huo et al. [33] evaluated the effect of hydrodynamic instability on flame acceleration. The unstable flame showed a pulsating acceleration process, and the acceleration process showed strong and weak periodic changes. Law et al. [34] and Kim et al. [35] evaluated the self-acceleration and self-similarity of a flame. Bauwens et al. [36] and others evaluated flame acceleration propagation under the combined action of turbulent flow and hydrodynamic instability and found that the flame develops self-similar oscillation. A turbulent flow can lead to the following phenomenon: The flame cellular structure appears ahead of time, the intermediate transition of the flame from laminar to self-similar oscillation becomes smoother, and the acceleration of flame becomes more intense after complete cell formation. Under different turbulent premixed flame modes, the influencing mechanisms of the turbulent burning velocity and flame acceleration propagation are different. To evaluate the propagation characteristics and flame self-acceleration characteristics of 10% H2/90% CO/air mixture under lean combustion in a turbulent environment, a series of experiments were carried out using a turbulent combustion system. For efficient and clean utilization of syngas, it is important to elucidate the influencing mechanism of the turbulent burning velocity and flame acceleration propagation of the syngas turbulent premixed flame under different turbulent premixed flame modes.
Experimental setup and procedures Experimental setup All the experiments were carried out in a spherical combustion bomb with an inner diameter of 380 mm. The experimental system consists of six parts: a constant-volume combustion bomb with a net volume of 28.73 L, a gas distribution system, an ignition system, a Schlieren system, an image acquisition system, and a data control and acquisition system. The combustion flame of syngas was captured using a high-speed camera at a shooting speed of 13,500 frames per second. The volume fraction of each component was calculated by Dalton’s mixing law, and syngas was added to the combustion bomb through the inlet valve. The ignition signal was provided using a synchronous trigger device, and the mixture was ignited using an ignition electrode installed in the horizontal position. At the same time, the high-speed camera was triggered to capture the image of the flame. A turbulent environment within the incendiary bomb was created using four fans and orifice plates arranged in a pyramid shape on the incendiary bomb. After the complete combustion of syngas/air mixture, the combustion products were pumped out using a vacuum pump and purged at least three times with clean air to eliminate the effect of residual combustion products on the experiment. The experimental setup is shown in Fig. 1. The turbulent premixed combustion flame images of 10% H2/90% CO/air mixture were captured by Schlieren-high-speed photography. The trend of turbulent premixed flame under
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Fig. 1 e Schematic of experimental setup. different initial conditions was accurately determined using self-coding MATLAB image processing program to extract and quantify the parameters of flame images. The basic information about the flame development such as radius, projection area, and contour circumference was obtained. The initial pressure and temperature were 0.1 MPa and 300 K, respectively. The turbulence intensity (u’) of the turbulent premixed combustion and the equivalence ratio (f) ranged from 0.20 m/s to 1.31 m/s and from 0.6 to 1.0, respectively. Due to the limitation of experimental equipment, the experiments were carried out using the flame images with the radius in the range 6e40 mm.
where Tad is the adiabatic flame temperature, Tu is the unburnt flame temperature, and (dT/dx)max is the maximum temperature gradient. The flame front surface area of the spherical flame changes continuously during flame propagation, thus changing the flame propagation speed. Flame stretch rate is expressed by the change rate of the flame front surface area to quantify the tensile action of the premixed flame and can be defined as follows:
Definition of parameters
K¼
The propagation speed of a turbulent flame is one of the most basic quantitative parameters of turbulent premixed flame and can be defined by Eq. (1). ST ¼
dRa dt
(1)
dL ¼
Tad Tu ðdT=dxÞmax
dðlnAÞ 1 dA ¼ dt A dt
(4)
(5)
The turbulent burning velocity (ut) can be defined by Eq. (6). ut ¼
ST ru =rb
(6)
where ru and rb are the unburned gas density and burned gas density, respectively.
where Ra is the area equivalence radius, defined as follows: 0:5 A Ra ¼ p
Calculation of acceleration exponent (2)
where A is the premixed flame surface area. The dimensionless treatment of flame radius is performed using the Pe number shown in Eq. (3). Pe ¼
Ra dL
where dL is the flame thickness calculated as follows:
(3)
The turbulent premixed flame of syngas/air mixture has three stages during its propagation: smooth expansion, transition, and saturation. For the smooth expansion stage, the speed of turbulent premixed flame propagation is limited, because of the high flame stretch rate in the smooth expansion stage of flame propagation. The flame front is smooth, or there is a small number of sparse cracks. With the rapid development of the flame front cellular structure and gradual decrease in
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flame stretch rate, the speed of turbulent premixed flame propagation increases rapidly. The flame starts to selfaccelerate and reaches the transition stage. At this stage, the cracks of the flame front develop rapidly, and the cells of the flame front divide rapidly (Fig. 2). With further development of flame, the development of a cellular structure of the flame front is limited, and the speed of the turbulent premixed flame propagation slows down gradually and the flame reaches the saturation stage. At this time, the flame front is covered with a uniform cellular structure. For syngas containing 10% H2, when the flame radius reaches 40 mm, the flame front is not covered with a uniform cell structure and the flame does not enter the saturation stage. During the development of a spherical premixed flame, the flame surface area significantly expanded because of the wrinkle and distortion of the flame front, leading to a sustained acceleration of flame spread. For a laminar premixed flame, the flame acceleration is caused by flame selfturbulence owing to the flame inherent instabilities. However, for a turbulent premixed flame, besides the flame inherent instabilities, the wrinkling effect of the turbulent flow on the flame front also significantly affects the accelerated propagation of the flame. The acceleration exponent a of the premixed expanded flame was quantified from the change in the flame radius with time as Ra~~ta . The greater the acceleration exponent, the more obvious the self-acceleration effect of the turbulent premixed flame. The flame propagation speed could be quantified as Sn~~Rda, and the fractal excess can be calculated by using equation d ¼ 1 1/a, which is usually used to assess the self-turbulent flame.
Results and discussion Propagation characteristics of turbulent premixed flame For a turbulent premixed flame, turbulence not only promotes the molecular diffusion between fuel and oxidizer, but also significantly wrinkles the flame front. Therefore, during the flame propagation, the increase in the turbulent burning
Fig. 2 e Turbulent premixed flame propagation characteristics of 10% H2/90% CO/air mixture.
velocity is mainly reflected by the remarkable increase in the flame radius with time. Fig. 3 shows the effect of turbulence intensity on the variation of flame radius with time at an equivalence ratio of 0.6. The change in the flame radius with time shows a linear trend in the smooth expansion stage of flame propagation. With further development of flame, the wrinkling of the flame front is enhanced, and the variation in the flame radius with time shows a nonlinear growth trend, indicating that the speed of turbulent premixed flame propagation increases gradually. As the turbulence intensity increases, the moment of the non-linear growth of the flame advances. In addition, with increasing turbulence intensity, the time required for flame propagation to the same radius decreases significantly. Fig. 4 shows the effect of different turbulence intensities on the propagation speed of 10% H2/90% CO/air mixture turbulent premixed flame at an equivalence ratio of 0.6. With the development of the flame, the growth rate of the turbulent premixed flame propagation speed gradually increases. As the turbulence intensity increased, the propagation speed of the syngas/air turbulent premixed flame showed an increasing trend, mainly because of the significant effect of turbulence on the development of flame structure. With increasing flow intensity, the disturbance of the turbulent premixed flame front enhances, and the evolution of the cellular structure of the flame front intensifies, increasing the turbulent premixed flame propagation speed. Fig. 5 shows the variation in the turbulent burning velocity of syngas/air mixture under different turbulent intensities at an equivalence ratio of 0.6. The effect of turbulence intensity on turbulent burning velocity is similar to that of turbulent premixed flame propagation speed. The growth rate of the turbulent combustion velocity increases with increasing turbulence intensity. With the development of the flame, the turbulent burning velocity showed a gradually increasing trend. When the turbulence intensity was 0.20 m/s, the turbulent burning velocity changed relatively smoothly because of relatively weak turbulence intensity. The greater the
Fig. 3 e Evolution of the flame radius for 10% H2/90% CO/air mixture under different turbulence intensities.
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Fig. 4 e Evolution of flame propagation speed for 10% H2/ 90% CO/air mixture under different turbulence intensities.
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Fig. 6 e Effect of equivalence ratio on the turbulent burning velocity of 10% H2/90% CO/air mixture. velocity showed a linear growth trend when the turbulence intensity increased from 0.20 m/s to 1.31 m/s. With the development of flame, the turbulent burning velocity of the syngas/air mixture showed an increasing trend at different equivalence ratios. In addition, the turbulent premixed flame of the syngas maintained a certain similarity during the propagation, and the trend of the turbulent burning velocity with turbulence intensity is similar to that of turbulent premixed flame propagation speed under different equivalence ratios. According to a previous study, turbulence affects the burning velocity of syngas/air mixture [40]. To further elucidate the mechanism of turbulent burning velocity, the relative turbulent burning velocity (the rate of the turbulent burning velocity ut to the laminar burning velocity ul) was selected to evaluate the effect of turbulence on the burning velocity of a syngas/air mixture. Fig. 7 shows the effect of equivalence ratio
Fig. 5 e Evolution of turbulent burning velocity for 10% H2/ 90% CO/air mixture under different turbulence intensities.
turbulent intensity, the greater the turbulent combustion velocity at the same radius. To evaluate the turbulent burning velocity of syngas/air mixture more clearly, further analyzing the turbulent burning velocity during flame propagation is necessary. When quantifying the turbulent combustion velocity of a spherical premixed flame, the turbulent combustion velocity at a flame radius of 30 mm has been often used in the literature [37e39]. The main reason is that when the flame propagates to 30 mm, the cellular structure of the flame front completely developed, and the effect of spark on the initial nucleus basically disappeared. Meanwhile, the turbulent burning velocity is not affected by the confined space pressure. Thus, the turbulent burning velocity was extracted and quantitatively analyzed at a flame radius of 30 mm. Fig. 6 shows the effect of turbulence intensity on the turbulent burning velocity of 10% H2/90% CO/ air mixture at a Ra of 30 mm. The overall turbulent burning
Fig. 7 e Effect of equivalence ratio on the relative turbulent burning velocity of 10% H2/90% CO/air mixture.
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Fig. 8 e Effect of u’/ul on the relative turbulent burning velocity of 10% H2/90% CO/air mixture.
on the relative turbulent burning velocity of 10% H2/90% CO/ air mixture at a Ra of 30 mm. With the increase of equivalence ratio, the relative turbulent burning velocity decreased as a whole, indicating that the effect of turbulence on turbulent burning velocity gradually decreases with increasing equivalence ratio. As the equivalence ratio increased, the thermal diffusion instability of syngas/air mixture increases, the rate of the flame front cellular formation decreases, the degree of the flame front wrinkle decreases, and the effect of the flame structure characteristics on the turbulent burning velocity decreases. Besides, the enhancement effect of turbulence intensity on the relative turbulent burning velocity of syngas/air mixture is quite different. The main reason is that both turbulence and flame inherent instability significantly affect the front structure of syngas/air turbulence premixed flame. At a relatively low equivalence ratio, the flame inherent instabilities has a relatively large effect on the flame front structure, leading to different turbulent burning velocities. Fig. 8 shows the plot of ut/ul against u’/ul of 10% H2/90% CO/ air mixture at a Ra of 30 mm. Clearly, u’/ul increases with increasing ut/ul. It was further confirmed that the turbulent
Fig. 9 e Effect of turbulence intensity on the evolution of 10% H2/90% CO/air mixture.
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burning velocity of a syngas/air mixture can be significantly enhanced at an appropriate turbulence intensity. However, the increase in turbulent burning velocity by increasing the turbulence intensity is not unlimited. With the increase of equivalence ratio, the magnitude of ut/ul growth gradually decreasing the effect of turbulence on the turbulent combustion rate of syngas/air mixture is significantly reduced, indicating that the flame inherent instabilities is equally important to the turbulent burning velocity.
Self-acceleration characteristics of turbulent premixed flame Fig. 9 shows the evolution of turbulent premixed flame of 10% H2/90% CO/air mixture at an equivalence ratio of 0.6. Clearly, cracks appear on the turbulent premixed flame front at the initial stage of propagation (Ra ¼ 10 mm). The cracks on the flame front cross form a cellular structure with increasing turbulence intensity to a certain extent. With further development of flame, the flame stretch rate acting on the flame front decreases gradually, and the disturbance of the flame front enhances gradually. The cracks on the flame front develop rapidly and form cellular structures covering the flame front. Because of the development of different scale disturbances on the flame front and the interaction between different scale disturbances, the wrinkle degree of the flame front is gradually intensified with the propagation of flame. As the turbulence intensity increased, the wrinkle degree of the flame front also increased gradually. When Ra propagated to 40 mm, the number of cells on the flame front increased significantly with increasing turbulence intensity, which enhances the size of the largest vortices acting on the flame front, and significantly increases the wrinkle degree of the flame front. Expanding flame is always affected by the flame stretch rate during its propagation. Compared to a laminar premixed flame, a spherical premixed flame is affected by both flame inherent instability and turbulent environment. With the development of flame, the flame stretch rate decreased rapidly as a power function. Fig. 10 shows the effect of turbulence intensity on the tensile strength of the turbulent premixed flame of syngas/air mixture at an equivalence ratio of 0.6. At the smooth expansion stage of flame propagation, the flame stretch rate decreased rapidly, and the change in the flame stretch rate slowed down gradually after flame propagation to a certain extent and gradually became stable. The tensile strength of the turbulent premixed flame of the syngas/air mixture with the same radius increased with increasing turbulence intensity. Fig. 11 shows the normalized flame speed vs. the Peclet number of 10% H2/90% CO/air mixture at an equivalence ratio of 0.6. A normalized flame propagation speed (ST/SL) can better explain the expanding in the flame surface area. With the increase of ST/SL, the flame became more wrinkled, thus increasing the flame surface area. Under the same turbulence intensity, the ST/SL increased with increasing Pe number. At low turbulence intensity, the growth rate of ST/SL increases. In addition, with increasing turbulence intensity, the Pe decreases with rapidly increasing ST/SL. At the same Pe number, ST/SL increased with increasing turbulence intensity. This also shows that an increase in turbulence intensity enhances the
Fig. 10 e Evolution of flame stretch rate of 10% H2/90%CO/ air mixture under different turbulence intensities.
instability of the turbulent premixed flame, thus increasing the wrinkle of the flame front and flame propagation speed. Fig. 12 shows the change in the acceleration exponent a and fractal excess d with equivalence ratio at a u’ of 1.08 m/s. In the transition stage, the a and d of the turbulent premixed flame of syngas/air mixture decreased with increasing equivalence ratio. The a of the turbulent premixed flame is greater than 1.5, indicating that the flame is in the selfacceleration state at this stage. As the equivalence ratio increased, the wrinkle of flame front decreases; thus, the acceleration exponent decreases nonlinearly with increasing equivalent ratio. Similarly, with the increase of equivalence ratio, the instability of flame structure decreases gradually, thus the fractal excess of turbulent premixed flame decreases nonlinearly. Fig. 13 shows the variation in the acceleration exponent and fractal excess with turbulence intensity at an equivalence
Fig. 11 e Normalized flame speed vs. the Peclet number of 10% H2/90% CO/air mixture at f ¼ 0.6.
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propagation characteristics of turbulent premixed flame and the self-acceleration characteristics of transition stage. The main conclusions of this study are as follows:
Fig. 12 e Fractal excess and acceleration exponent of turbulent premixed flame in 10% H2/90% CO/air mixture under different equivalence ratios.
Fig. 13 e Fractal excess and acceleration exponent of turbulent premixed flame in 10% H2/90% CO/air mixture under different turbulence intensities.
ratio of 0.6. As the turbulence intensity increased, the a and d values of the turbulent premixed flame of syngas/air mixture increase. For the premixed flame of syngas/air mixture at an equivalence ratio of 0.6, the thermal diffusion instability is stronger, leading to the nonlinear growth of the acceleration exponent. This also leads to a nonlinear increase in the fractal excess of the turbulent premixed flame with increasing turbulence intensity. Meanwhile, the acceleration exponent of the turbulent premixed flame in the transition stage of flame propagation is greater than 1.5, indicating that the flame is in a complete self-acceleration state at this time.
Conclusions The effects of the equivalence ratio and turbulence intensity on the nonlinear propagation characteristics of the turbulent premixed flame of a syngas/air mixture were evaluated by the
1. Turbulence gradually changes the radius of the premixed flame from linear growth to nonlinear growth and shortens the time required for propagation. With the increase of turbulence intensity, the disturbance of the turbulent premixed flame front increases, the evolution of the flame front cell structure intensifies, and the wrinkle degree of flame front increases, thus increasing the turbulent premixed flame propagation speed. 2. As the turbulence intensity increased, the turbulent combustion rate showed an increasing growth trend. The trend of the turbulent burning velocity with turbulence intensity is similar to that of the turbulent premixed flame propagation velocity. With the increase of equivalence ratio, the relative turbulent burning velocity decreases as a whole. With the increase of turbulence intensity, the relative turbulence combustion velocity of syngas/air mixture increases at the same equivalence ratio. 3. With the increase of turbulence intensity, the size of the largest vortices acting on the flame front increases, and the wrinkle degree of flame front increases significantly, and the tensile strength of the turbulent premixed flame of the syngas/air mixture with the same radius increases. At the same turbulence intensity, the ST/SL increases with increasing Pe number. At the same Pe number, the ST/SL increases with increasing turbulence intensity. 4. In the transition stage, the a and d values of the turbulent premixed flame of syngas/air mixture decrease with increasing equivalence ratio. The acceleration exponent of the turbulent premixed flame is greater than 1.5 at a turbulence intensity of 1.08 m/s. The a and d values of turbulent premixed flame of the syngas/air mixture increase with the increase in turbulence intensity. The acceleration exponents of the turbulent premixed flame in the transition stage of the flame propagation are greater than 1.5 at an equivalence ratio of 0.6, indicating that the flame is in a completely self-accelerating state.
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51706014), the Fundamental Research Funds for the Central Universities (No. 2017JBZ102).
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Experimental investigation on the self-acceleration of 10%H2/90%CO/air turbulent expanding premixed flame Guo-Peng Zhang a,b, Guo-Xiu Li a,b,*, Hong-Meng Li a,b, Yan-Huan Jiang a,b, Jia-Cheng Lv a,b a
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing, 100044, China Key Laboratory of Vehicle Advanced Manufacturing, Measuring and Control Technology (Beijing Jiaotong University), Ministry of Education, Beijing, 100044, China
b
highlights Effect of turbulence intensity on turbulent burning velocity was studied. Nonlinear propagation characteristics of turbulent premixed flame were evaluated. Fractal and acceleration characteristics in the transition stage were studied.
article info
abstract
Article history:
The self-acceleration characteristics of a syngas/air mixture turbulent premixed flame
Received 21 May 2019
were experimentally evaluated using a 10% H2/90% CO/air mixture turbulent premixed
Received in revised form
flame by varying the turbulence intensity and equivalence ratio at atmospheric pressure
19 July 2019
and temperature. The propagation characteristics of the turbulent premixed flame
Accepted 22 July 2019
including the variation in the flame propagation speed and turbulent burning velocity of
Available online 10 August 2019
the syngas/air mixture turbulent premixed flame were evaluated. In addition, the effect of the self-acceleration characteristics of the turbulent premixed flame was also evaluated.
Keywords:
The results show that turbulence gradually changes the radius of the premixed flame from
Syngas
linear growth to nonlinear growth. With the increase of turbulence intensity, the formation
Turbulent premixed flame
of a cellular structure of the flame front accelerated, increasing the flame propagation
Flame propagation speed
speed and burning speed. In the transition stage, the acceleration exponent and fractal
Turbulent burning velocity
excess of the turbulent premixed flame decreased with increasing equivalence ratio and
Self-acceleration characteristic
increased with increasing turbulence intensity at an equivalence ratio of 0.6. The acceleration exponent was always greater than 1.5. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China. E-mail address: [email protected] (G.-X. Li). https://doi.org/10.1016/j.ijhydene.2019.07.154 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction In view of increasingly severe environmental problems and energy shortages, two important solutions are the exploration of clean and reliable alternative energy sources and the elucidation of combustion mechanisms to achieve efficient fuel combustion [1e6]. Syngas, a mixture of CO and H2 as the main components, has many sources and can be produced by a variety of methods. It can be prepared by the gasification of a solid fuel such as coal or coke [7], or light hydrocarbons such as methane and propane [8]. It can also be partially oxidized with macromolecular hydrocarbon fuels such as diesel and kerosene [9]. As an important component of syngas, H2 has the advantages of fast flame propagation, low ignition energy, and wide combustion limit [10]. Therefore, syngas has been widely investigated as an alternative fuel for internal combustion engines [11e13]. The turbulent expanding premixed flame is a major form of combustion and a typical form of flame in the spark-ignited internal combustion engines [14]. The turbulent expanding premixed flame has been investigated in detail [15,16]. Different turbulent expanding premixed flame combustion characteristics have been proposed, because of the difference in fuel [17], turbulence intensity [18], equivalence ratio [19], and the range of observation radius [20]. Gas-fueled engine suffers from combustion cycle variation. Studies have shown that the combustion cycle variation of an engine is mainly caused by a turbulent flow acting on flame propagation [21]. Therefore, it is important to elucidate the mechanism of the propagation characteristics of the turbulent premixed flame of syngas for optimizing the combustion process of power machinery and design of combustion system. Pierre et al. [22] evaluated the effect of turbulence intensity and pressure on the flame propagation speed in a spherical turbulent premixed flame and used them as a radius or stretch function to better understand the dynamic characteristics of flame propagation. Li et al. [23] evaluated the distribution of the local radius of flame and the local propagation speed of flame at different turbulence intensities of an equimolar mixture of H2 and CO and elucidated their relationship. Turbulence is known to increase the disturbance of the flame front and enhance the fluctuation of local radius of flame, thus increasing the fluctuation in the local propagation speed of the flame. Goulier et al. [24] reported the effect of the turbulent flame speed correlation on the turbulent combustion characteristics of a lean to stoichiometric H2/air mixture. In addition, Wang et al. [25] evaluated the correlation of turbulent burning velocity under a high pressure of syngas premixed flame. Chiu et al. [26] studied the turbulent burning velocity of a highpressure syngas under constant Reynolds number. A turbulent premixed flame undergoes an accelerated propagation process. On one hand, the flame inherent instabilities of a flame makes the flame front produce a cellular structure and accelerate flame propagation. On the other hand, the scale of turbulent flow effect on flame becomes larger, turbulent disturbance is enhanced, and flame propagation is accelerated. At present, many studies do not consider turbulent flow, but only focus on the effect of flame inherent instability on accelerated flame propagation [27e31]. Yang et al. [32] evaluated the effect of flame inherent instability on the flame
propagation and self-acceleration in a laminar flow environment. The formation of a cellular structure on the flame front is dominated by thermal diffusive instability, and the flame front develops to saturation region by hydrodynamic instability. Huo et al. [33] evaluated the effect of hydrodynamic instability on flame acceleration. The unstable flame showed a pulsating acceleration process, and the acceleration process showed strong and weak periodic changes. Law et al. [34] and Kim et al. [35] evaluated the self-acceleration and self-similarity of a flame. Bauwens et al. [36] and others evaluated flame acceleration propagation under the combined action of turbulent flow and hydrodynamic instability and found that the flame develops self-similar oscillation. A turbulent flow can lead to the following phenomenon: The flame cellular structure appears ahead of time, the intermediate transition of the flame from laminar to self-similar oscillation becomes smoother, and the acceleration of flame becomes more intense after complete cell formation. Under different turbulent premixed flame modes, the influencing mechanisms of the turbulent burning velocity and flame acceleration propagation are different. To evaluate the propagation characteristics and flame self-acceleration characteristics of 10% H2/90% CO/air mixture under lean combustion in a turbulent environment, a series of experiments were carried out using a turbulent combustion system. For efficient and clean utilization of syngas, it is important to elucidate the influencing mechanism of the turbulent burning velocity and flame acceleration propagation of the syngas turbulent premixed flame under different turbulent premixed flame modes.
Experimental setup and procedures Experimental setup All the experiments were carried out in a spherical combustion bomb with an inner diameter of 380 mm. The experimental system consists of six parts: a constant-volume combustion bomb with a net volume of 28.73 L, a gas distribution system, an ignition system, a Schlieren system, an image acquisition system, and a data control and acquisition system. The combustion flame of syngas was captured using a high-speed camera at a shooting speed of 13,500 frames per second. The volume fraction of each component was calculated by Dalton’s mixing law, and syngas was added to the combustion bomb through the inlet valve. The ignition signal was provided using a synchronous trigger device, and the mixture was ignited using an ignition electrode installed in the horizontal position. At the same time, the high-speed camera was triggered to capture the image of the flame. A turbulent environment within the incendiary bomb was created using four fans and orifice plates arranged in a pyramid shape on the incendiary bomb. After the complete combustion of syngas/air mixture, the combustion products were pumped out using a vacuum pump and purged at least three times with clean air to eliminate the effect of residual combustion products on the experiment. The experimental setup is shown in Fig. 1. The turbulent premixed combustion flame images of 10% H2/90% CO/air mixture were captured by Schlieren-high-speed photography. The trend of turbulent premixed flame under
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Fig. 1 e Schematic of experimental setup. different initial conditions was accurately determined using self-coding MATLAB image processing program to extract and quantify the parameters of flame images. The basic information about the flame development such as radius, projection area, and contour circumference was obtained. The initial pressure and temperature were 0.1 MPa and 300 K, respectively. The turbulence intensity (u’) of the turbulent premixed combustion and the equivalence ratio (f) ranged from 0.20 m/s to 1.31 m/s and from 0.6 to 1.0, respectively. Due to the limitation of experimental equipment, the experiments were carried out using the flame images with the radius in the range 6e40 mm.
where Tad is the adiabatic flame temperature, Tu is the unburnt flame temperature, and (dT/dx)max is the maximum temperature gradient. The flame front surface area of the spherical flame changes continuously during flame propagation, thus changing the flame propagation speed. Flame stretch rate is expressed by the change rate of the flame front surface area to quantify the tensile action of the premixed flame and can be defined as follows:
Definition of parameters
K¼
The propagation speed of a turbulent flame is one of the most basic quantitative parameters of turbulent premixed flame and can be defined by Eq. (1). ST ¼
dRa dt
(1)
dL ¼
Tad Tu ðdT=dxÞmax
dðlnAÞ 1 dA ¼ dt A dt
(4)
(5)
The turbulent burning velocity (ut) can be defined by Eq. (6). ut ¼
ST ru =rb
(6)
where ru and rb are the unburned gas density and burned gas density, respectively.
where Ra is the area equivalence radius, defined as follows: 0:5 A Ra ¼ p
Calculation of acceleration exponent (2)
where A is the premixed flame surface area. The dimensionless treatment of flame radius is performed using the Pe number shown in Eq. (3). Pe ¼
Ra dL
where dL is the flame thickness calculated as follows:
(3)
The turbulent premixed flame of syngas/air mixture has three stages during its propagation: smooth expansion, transition, and saturation. For the smooth expansion stage, the speed of turbulent premixed flame propagation is limited, because of the high flame stretch rate in the smooth expansion stage of flame propagation. The flame front is smooth, or there is a small number of sparse cracks. With the rapid development of the flame front cellular structure and gradual decrease in
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flame stretch rate, the speed of turbulent premixed flame propagation increases rapidly. The flame starts to selfaccelerate and reaches the transition stage. At this stage, the cracks of the flame front develop rapidly, and the cells of the flame front divide rapidly (Fig. 2). With further development of flame, the development of a cellular structure of the flame front is limited, and the speed of the turbulent premixed flame propagation slows down gradually and the flame reaches the saturation stage. At this time, the flame front is covered with a uniform cellular structure. For syngas containing 10% H2, when the flame radius reaches 40 mm, the flame front is not covered with a uniform cell structure and the flame does not enter the saturation stage. During the development of a spherical premixed flame, the flame surface area significantly expanded because of the wrinkle and distortion of the flame front, leading to a sustained acceleration of flame spread. For a laminar premixed flame, the flame acceleration is caused by flame selfturbulence owing to the flame inherent instabilities. However, for a turbulent premixed flame, besides the flame inherent instabilities, the wrinkling effect of the turbulent flow on the flame front also significantly affects the accelerated propagation of the flame. The acceleration exponent a of the premixed expanded flame was quantified from the change in the flame radius with time as Ra~~ta . The greater the acceleration exponent, the more obvious the self-acceleration effect of the turbulent premixed flame. The flame propagation speed could be quantified as Sn~~Rda, and the fractal excess can be calculated by using equation d ¼ 1 1/a, which is usually used to assess the self-turbulent flame.
Results and discussion Propagation characteristics of turbulent premixed flame For a turbulent premixed flame, turbulence not only promotes the molecular diffusion between fuel and oxidizer, but also significantly wrinkles the flame front. Therefore, during the flame propagation, the increase in the turbulent burning
Fig. 2 e Turbulent premixed flame propagation characteristics of 10% H2/90% CO/air mixture.
velocity is mainly reflected by the remarkable increase in the flame radius with time. Fig. 3 shows the effect of turbulence intensity on the variation of flame radius with time at an equivalence ratio of 0.6. The change in the flame radius with time shows a linear trend in the smooth expansion stage of flame propagation. With further development of flame, the wrinkling of the flame front is enhanced, and the variation in the flame radius with time shows a nonlinear growth trend, indicating that the speed of turbulent premixed flame propagation increases gradually. As the turbulence intensity increases, the moment of the non-linear growth of the flame advances. In addition, with increasing turbulence intensity, the time required for flame propagation to the same radius decreases significantly. Fig. 4 shows the effect of different turbulence intensities on the propagation speed of 10% H2/90% CO/air mixture turbulent premixed flame at an equivalence ratio of 0.6. With the development of the flame, the growth rate of the turbulent premixed flame propagation speed gradually increases. As the turbulence intensity increased, the propagation speed of the syngas/air turbulent premixed flame showed an increasing trend, mainly because of the significant effect of turbulence on the development of flame structure. With increasing flow intensity, the disturbance of the turbulent premixed flame front enhances, and the evolution of the cellular structure of the flame front intensifies, increasing the turbulent premixed flame propagation speed. Fig. 5 shows the variation in the turbulent burning velocity of syngas/air mixture under different turbulent intensities at an equivalence ratio of 0.6. The effect of turbulence intensity on turbulent burning velocity is similar to that of turbulent premixed flame propagation speed. The growth rate of the turbulent combustion velocity increases with increasing turbulence intensity. With the development of the flame, the turbulent burning velocity showed a gradually increasing trend. When the turbulence intensity was 0.20 m/s, the turbulent burning velocity changed relatively smoothly because of relatively weak turbulence intensity. The greater the
Fig. 3 e Evolution of the flame radius for 10% H2/90% CO/air mixture under different turbulence intensities.
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Fig. 4 e Evolution of flame propagation speed for 10% H2/ 90% CO/air mixture under different turbulence intensities.
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Fig. 6 e Effect of equivalence ratio on the turbulent burning velocity of 10% H2/90% CO/air mixture. velocity showed a linear growth trend when the turbulence intensity increased from 0.20 m/s to 1.31 m/s. With the development of flame, the turbulent burning velocity of the syngas/air mixture showed an increasing trend at different equivalence ratios. In addition, the turbulent premixed flame of the syngas maintained a certain similarity during the propagation, and the trend of the turbulent burning velocity with turbulence intensity is similar to that of turbulent premixed flame propagation speed under different equivalence ratios. According to a previous study, turbulence affects the burning velocity of syngas/air mixture [40]. To further elucidate the mechanism of turbulent burning velocity, the relative turbulent burning velocity (the rate of the turbulent burning velocity ut to the laminar burning velocity ul) was selected to evaluate the effect of turbulence on the burning velocity of a syngas/air mixture. Fig. 7 shows the effect of equivalence ratio
Fig. 5 e Evolution of turbulent burning velocity for 10% H2/ 90% CO/air mixture under different turbulence intensities.
turbulent intensity, the greater the turbulent combustion velocity at the same radius. To evaluate the turbulent burning velocity of syngas/air mixture more clearly, further analyzing the turbulent burning velocity during flame propagation is necessary. When quantifying the turbulent combustion velocity of a spherical premixed flame, the turbulent combustion velocity at a flame radius of 30 mm has been often used in the literature [37e39]. The main reason is that when the flame propagates to 30 mm, the cellular structure of the flame front completely developed, and the effect of spark on the initial nucleus basically disappeared. Meanwhile, the turbulent burning velocity is not affected by the confined space pressure. Thus, the turbulent burning velocity was extracted and quantitatively analyzed at a flame radius of 30 mm. Fig. 6 shows the effect of turbulence intensity on the turbulent burning velocity of 10% H2/90% CO/ air mixture at a Ra of 30 mm. The overall turbulent burning
Fig. 7 e Effect of equivalence ratio on the relative turbulent burning velocity of 10% H2/90% CO/air mixture.
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Fig. 8 e Effect of u’/ul on the relative turbulent burning velocity of 10% H2/90% CO/air mixture.
on the relative turbulent burning velocity of 10% H2/90% CO/ air mixture at a Ra of 30 mm. With the increase of equivalence ratio, the relative turbulent burning velocity decreased as a whole, indicating that the effect of turbulence on turbulent burning velocity gradually decreases with increasing equivalence ratio. As the equivalence ratio increased, the thermal diffusion instability of syngas/air mixture increases, the rate of the flame front cellular formation decreases, the degree of the flame front wrinkle decreases, and the effect of the flame structure characteristics on the turbulent burning velocity decreases. Besides, the enhancement effect of turbulence intensity on the relative turbulent burning velocity of syngas/air mixture is quite different. The main reason is that both turbulence and flame inherent instability significantly affect the front structure of syngas/air turbulence premixed flame. At a relatively low equivalence ratio, the flame inherent instabilities has a relatively large effect on the flame front structure, leading to different turbulent burning velocities. Fig. 8 shows the plot of ut/ul against u’/ul of 10% H2/90% CO/ air mixture at a Ra of 30 mm. Clearly, u’/ul increases with increasing ut/ul. It was further confirmed that the turbulent
Fig. 9 e Effect of turbulence intensity on the evolution of 10% H2/90% CO/air mixture.
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burning velocity of a syngas/air mixture can be significantly enhanced at an appropriate turbulence intensity. However, the increase in turbulent burning velocity by increasing the turbulence intensity is not unlimited. With the increase of equivalence ratio, the magnitude of ut/ul growth gradually decreasing the effect of turbulence on the turbulent combustion rate of syngas/air mixture is significantly reduced, indicating that the flame inherent instabilities is equally important to the turbulent burning velocity.
Self-acceleration characteristics of turbulent premixed flame Fig. 9 shows the evolution of turbulent premixed flame of 10% H2/90% CO/air mixture at an equivalence ratio of 0.6. Clearly, cracks appear on the turbulent premixed flame front at the initial stage of propagation (Ra ¼ 10 mm). The cracks on the flame front cross form a cellular structure with increasing turbulence intensity to a certain extent. With further development of flame, the flame stretch rate acting on the flame front decreases gradually, and the disturbance of the flame front enhances gradually. The cracks on the flame front develop rapidly and form cellular structures covering the flame front. Because of the development of different scale disturbances on the flame front and the interaction between different scale disturbances, the wrinkle degree of the flame front is gradually intensified with the propagation of flame. As the turbulence intensity increased, the wrinkle degree of the flame front also increased gradually. When Ra propagated to 40 mm, the number of cells on the flame front increased significantly with increasing turbulence intensity, which enhances the size of the largest vortices acting on the flame front, and significantly increases the wrinkle degree of the flame front. Expanding flame is always affected by the flame stretch rate during its propagation. Compared to a laminar premixed flame, a spherical premixed flame is affected by both flame inherent instability and turbulent environment. With the development of flame, the flame stretch rate decreased rapidly as a power function. Fig. 10 shows the effect of turbulence intensity on the tensile strength of the turbulent premixed flame of syngas/air mixture at an equivalence ratio of 0.6. At the smooth expansion stage of flame propagation, the flame stretch rate decreased rapidly, and the change in the flame stretch rate slowed down gradually after flame propagation to a certain extent and gradually became stable. The tensile strength of the turbulent premixed flame of the syngas/air mixture with the same radius increased with increasing turbulence intensity. Fig. 11 shows the normalized flame speed vs. the Peclet number of 10% H2/90% CO/air mixture at an equivalence ratio of 0.6. A normalized flame propagation speed (ST/SL) can better explain the expanding in the flame surface area. With the increase of ST/SL, the flame became more wrinkled, thus increasing the flame surface area. Under the same turbulence intensity, the ST/SL increased with increasing Pe number. At low turbulence intensity, the growth rate of ST/SL increases. In addition, with increasing turbulence intensity, the Pe decreases with rapidly increasing ST/SL. At the same Pe number, ST/SL increased with increasing turbulence intensity. This also shows that an increase in turbulence intensity enhances the
Fig. 10 e Evolution of flame stretch rate of 10% H2/90%CO/ air mixture under different turbulence intensities.
instability of the turbulent premixed flame, thus increasing the wrinkle of the flame front and flame propagation speed. Fig. 12 shows the change in the acceleration exponent a and fractal excess d with equivalence ratio at a u’ of 1.08 m/s. In the transition stage, the a and d of the turbulent premixed flame of syngas/air mixture decreased with increasing equivalence ratio. The a of the turbulent premixed flame is greater than 1.5, indicating that the flame is in the selfacceleration state at this stage. As the equivalence ratio increased, the wrinkle of flame front decreases; thus, the acceleration exponent decreases nonlinearly with increasing equivalent ratio. Similarly, with the increase of equivalence ratio, the instability of flame structure decreases gradually, thus the fractal excess of turbulent premixed flame decreases nonlinearly. Fig. 13 shows the variation in the acceleration exponent and fractal excess with turbulence intensity at an equivalence
Fig. 11 e Normalized flame speed vs. the Peclet number of 10% H2/90% CO/air mixture at f ¼ 0.6.
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propagation characteristics of turbulent premixed flame and the self-acceleration characteristics of transition stage. The main conclusions of this study are as follows:
Fig. 12 e Fractal excess and acceleration exponent of turbulent premixed flame in 10% H2/90% CO/air mixture under different equivalence ratios.
Fig. 13 e Fractal excess and acceleration exponent of turbulent premixed flame in 10% H2/90% CO/air mixture under different turbulence intensities.
ratio of 0.6. As the turbulence intensity increased, the a and d values of the turbulent premixed flame of syngas/air mixture increase. For the premixed flame of syngas/air mixture at an equivalence ratio of 0.6, the thermal diffusion instability is stronger, leading to the nonlinear growth of the acceleration exponent. This also leads to a nonlinear increase in the fractal excess of the turbulent premixed flame with increasing turbulence intensity. Meanwhile, the acceleration exponent of the turbulent premixed flame in the transition stage of flame propagation is greater than 1.5, indicating that the flame is in a complete self-acceleration state at this time.
Conclusions The effects of the equivalence ratio and turbulence intensity on the nonlinear propagation characteristics of the turbulent premixed flame of a syngas/air mixture were evaluated by the
1. Turbulence gradually changes the radius of the premixed flame from linear growth to nonlinear growth and shortens the time required for propagation. With the increase of turbulence intensity, the disturbance of the turbulent premixed flame front increases, the evolution of the flame front cell structure intensifies, and the wrinkle degree of flame front increases, thus increasing the turbulent premixed flame propagation speed. 2. As the turbulence intensity increased, the turbulent combustion rate showed an increasing growth trend. The trend of the turbulent burning velocity with turbulence intensity is similar to that of the turbulent premixed flame propagation velocity. With the increase of equivalence ratio, the relative turbulent burning velocity decreases as a whole. With the increase of turbulence intensity, the relative turbulence combustion velocity of syngas/air mixture increases at the same equivalence ratio. 3. With the increase of turbulence intensity, the size of the largest vortices acting on the flame front increases, and the wrinkle degree of flame front increases significantly, and the tensile strength of the turbulent premixed flame of the syngas/air mixture with the same radius increases. At the same turbulence intensity, the ST/SL increases with increasing Pe number. At the same Pe number, the ST/SL increases with increasing turbulence intensity. 4. In the transition stage, the a and d values of the turbulent premixed flame of syngas/air mixture decrease with increasing equivalence ratio. The acceleration exponent of the turbulent premixed flame is greater than 1.5 at a turbulence intensity of 1.08 m/s. The a and d values of turbulent premixed flame of the syngas/air mixture increase with the increase in turbulence intensity. The acceleration exponents of the turbulent premixed flame in the transition stage of the flame propagation are greater than 1.5 at an equivalence ratio of 0.6, indicating that the flame is in a completely self-accelerating state.
Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51706014), the Fundamental Research Funds for the Central Universities (No. 2017JBZ102).
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